1. Field of the Invention:
The present invention relates to a solenoid operated valve apparatus, and, more particularly, to a solenoid device of a solenoid operated valve apparatus used for controlling the flow rate of a hydraulic fluid.
2. Description of the Related Art:
Conventional solenoid devices of solenoid operated valve apparatus are disclosed in, for example, JP-A-47-22516. The solenoid device of this type includes an inner yoke, a plunger guide disposed in opposed relationship with the inner yoke, and a plunger movably provided in a space or a plunger chamber formed between the inner yoke and the plunger guide. A lid body is fitted in the inner yoke. First and second rods of the plunger are movably supported by a bearing fitted in the plunger guide and a bearing fitted in the lid body, respectively. The solenoid device also includes a non-magnetic ring fitted on the plunger guide and inner yoke through O-rings to surround portions of the plunger guide and the inner yoke, a bobbin frame fitted on the non-magnetic ring to surround the same, a coil wound around the bobbin frame, and an outer yoke fitted on the bobbin frame to surround the coil. The plunger guide is provided with a communication hole through which hydraulic fluid flows into and out of the plunger chamber.
In the thus-arranged solenoid device, the plunger moves by virtue of a magnetic field formed when the coil is excited, thereby moving a spool of a solenoid operated valve body (not shown) which is in contact with the first rod. In consequence, the opening of a variable restricting portion associated with the spool, for example, is controlled to accomplish a predetermined valve function, e.g., flow rate control or pressure control.
The precision with which the above-described solenoid device operates depends on the precision with which it is assembled. This means that the precision of the device in operation depends on the accuracy of the centering between the plunger guide and the lid body which supports the plunger. Since the lid body is fitted in the inner yoke, the precision of the device further relies on the accuracy of the centering between the plunger guide and the inner yoke. However, in the conventional solenoid device, since the non-magnetic ring is mounted to surround the plunger guide and the inner yoke and the bobbin frame is in turn mounted to surround this non-magnetic ring, the number of man-hours required for machining and assembling the parts must be high for achieving a desired centering accuracy, thereby increasing the overall production cost.
More specifically, the accuracy of the centering between the plunger guide and the inner yoke relies on the machining precision when these parts are made. Further, the non-magnetic ring is thin and therefore cannot be made strong enough to withstand the force of a bobbin frame bearing against it if the latter is misplaced from the center position. Thus any misalignment of the bobbin frame affects the plunger guide and the inner yoke. This means that the accuracy with which the centering is performed also depends on the machining precision of the non-magnetic ring and the bobbin frame. Thus, in order to improve the centering accuracy, these four parts must be machined with a high degree of precision and be assembled with the greatest possible care.
Further, in the conventional solenoid device of the above-described type, since the inner yoke, the plunger guide and the non-magnetic ring are provided as separate members, O-rings must be provided so as to prevent oil leakage from the gap between these members, thereby increasing the number of parts required and, hence, the production cost.